Barrier films for plastic substrates fabricated by atomic layer deposition

ABSTRACT

Gas permeation barriers can be deposited on plastic or glass substrates by atomic layer deposition (ALD). The use of the ALD coatings can reduce permeation by many orders of magnitude at thicknesses of tens of nanometers with low concentrations of coating defects. These thin coatings preserve the flexibility and transparency of the plastic substrate. Such articles are useful in container, electrical and electronic applications.

FIELD OF THE INVENTION

The present invention relates to an article comprising a plastic orglass substrate and an atmospheric gas penetration barrier fabricated byatomic layer deposition. The article may be a component of an electricalor electronic device such as an organic light emitting diode. Thearticle may also be used as a container for applications where gaspermeation is important.

TECHNICAL BACKGROUND

Featherby and Dehaven (WO 2001067504) disclose a hermetically coateddevice. Formation of such a device includes the steps of providing anintegrated semiconductor circuit die, applying a first layer comprisingan inorganic material which envelopes the circuit die, and applying asecond layer enveloping the circuit die.

Aintila (WO 9715070 A2) discloses contact bump formation on metalliccontact pad areas on the surface of a substrate comprising using atomiclayer epitaxy to form an oxide layer on the substrate which is opened atrequired points in the subsequent process step.

Aftergut and Ackerman (U.S. Pat. No. 5,641,984) disclose a hermeticallypackaged radiation imager including a moisture barrier. A dielectricmaterial layer is deposited in an atomic layer expitaxy technique aspart of the sealing structure.

Aftergut and Ackerman (U.S. Pat. No. 5,707,880) disclose a hermeticallypackaged radiation imager including a moisture barrier comprising adielectric material layer deposited by atomic layer expitaxy.

None of the references disclosed a permeation barrier comprising apolymer or glass substrate.

SUMMARY OF THE INVENTION

This invention describes an article comprising:

-   -   a) a substrate made of a material selected from the group        consisting of plastic and glass, and    -   b) a film deposited upon said substrate by atomic layer        deposition.    -   The present invention is further an article comprising:    -   a) A substrate made of a material selected from the group        consisting of plastic and glass;    -   b) an adhesion layer coated; and    -   c) a gas permeation barrier deposited by atomic layer        deposition.        Another embodiment of the present invention is an article        comprising:    -   a) a substrate made of a material selected from the group        consisting of plastic and glass;    -   b) an organic semiconductor, and    -   c) a gas permeation barrier deposited by atomic layer        deposition.        A yet further embodiment of the present invention is an article        comprising:    -   a) A substrate made of a material selected from the group        consisting of plastic and glass,    -   b) A liquid crystal polymer, and    -   c) a gas permeation barrier deposited by atomic layer deposition

The invention further describes an embodiment that is an enclosedcontainer.

Another embodiment of the present invention is an electrical orelectronic device.

Yet another embodiment of the present invention is a light-emittingpolymer device.

Yet another embodiment of the present invention is liquid crystallinepolymer device.

The invention further describes an organic light emitting diode.

Another embodiment of the present invention is a transistor.

Yet another embodiment of the present invention is a circuit comprisinga light emitting polymer device.

A still further article is an organic photovoltaic cell.

A second article taught herein comprises a plurality of layers, eachlayer comprising one article, as described above, wherein the articlesare in contact with each other. In one embodiment of this second articleof the articles above are in contact with each other by laminationmeans.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a light-emitting polymer device with a barrier substrateand a barrier top coat.

FIG. 2 shows a light-emitting polymer device with a barrier substrateand a barrier capping layer.

FIG. 3 shows an organic transistor with a barrier substrate and abarrier capping layer.

FIG. 4 shows an organic transistor with a barrier substrate and abarrier capping layer.

FIG. 5 shows the measured optical transmission through 0.002 inch thickpolyethylene naphthalate (PEN) coated with 25 nm of Al₂O₃ barrier film.

DETAILED DESCRIPTION

The permeation of O₂ and H₂O vapor through polymer films is facile. Toreduce permeability for packaging applications, polymers are coated witha thin inorganic film. Al-coated polyester is common. Opticallytransparent barriers, predominantly SiOx or AlO_(y), made either byphysical vapor deposition (PVD) or chemical vapor deposition (CVD), arealso used in packaging. The latter films are commercially available andare known in the industry as “glass-coated” barrier films. They providean improvement for atmospheric gas permeation of about 10×, reducingtransmission rates to about 1.0 cc O₂/m²/day and 1.0 ml H₂O/m²/daythrough polyester film (M. Izu, B. Dotter, and S. R. Ovshinsky, J.Photopolymer Science and Technology., vol. 8 1995 pp 195-204). Whilethis modest improvement is a reasonable compromise between performanceand cost for many high-volume packaging applications, this performancefalls far short of packaging requirements in electronics. Electronicpackaging usually requires at least an order of magnitude longer desiredlifetime than, for example, beverage containing. As an example, flexibledisplays based on organic light emitting polymers (OLEDs), fabricated onflexible polyester substrates need an estimated barrier improvement of10⁵-10⁶× for exclusion of atmospheric gases since gases can seriouslydegrade both the light-emitting polymer and the water-sensitive metalcathode which can frequently be Ca or Ba.

Because of their inherent free volume fraction, the intrinsicpermeability of polymers is, in general, too high by a factor 10⁴-10⁶ toachieve the level of protection needed in electronic applications, suchas flexible OLED displays. Only inorganic materials, with essentiallyzero permeability, can provide adequate barrier protection. Ideally, adefect-free, continuous thin-film coating of an inorganic should beimpermeable to atmospheric gases. However, the practical reality is thatthin films have defects, such as pinholes, either from the coatingprocess or from substrate imperfections which compromise barrierproperties. Even grain boundaries in films can present a pathway forfacile permeation. For the best barrier properties, films should bedeposited in a clean environment on clean, defect-free substrates. Thefilm structure should be amorphous. The deposition process should benon-directional, (i.e. CVD is preferred over PVD) and the growthmechanism to achieve a featureless microstructure would ideally belayer-by-layer to avoid columnar growth with granular microstructure.

Atomic layer deposition (ALD) is a film growth method that satisfiesmany of these criteria for low permeation. A description of the atomiclayer deposition process can be found in “Atomic Layer Epitaxy,” byTuomo Suntola in Thin Solid Films, vol. 216 (1992) pp. 84-89. As itsname implies, films grown by ALD form by a layer by layer process. Ingeneral, a vapor of film precursor is absorbed on a substrate in avacuum chamber. The vapor is then pumped from the chamber, leaving athin layer of absorbed precursor, usually essentially a monolayer, onthe substrate. A reactant is then introduced into the chamber underthermal conditions, which promote reaction with the absorbed precursorto form a layer of the desired material. The reaction products arepumped from the chamber. Subsequent layers of material can be formed byagain exposing the substrate to the precursor vapor and repeating thedeposition process. ALD is in contrast to growth by common CVD and PVDmethods where growth is initiated and proceeds at finite numbers ofnucleation sites on the substrate surface. The latter technique can leadto a columnar microstructures with boundaries between columns alongwhich gas permeation can be facile. ALD can produce very thin films withextremely low gas permeability, making such films attractive as barrierlayers for packaging sensitive electronic devices and components builton plastic substrates.

This invention describes barrier layers formed by ALD on plasticsubstrates and useful for preventing the passage of atmospheric gases.The substrates of this invention include the general class of polymericmaterials, such as described by but not limited to those in PolymerMaterials, (Wiley, New York, 1989) by Christopher Hall or PolymerPermeability, (Elsevier, London, 1985) by J. Comyn. Common examplesinclude polyethylene terephthalate (PET) and polyethylene naphthalate(PEN), which are commercially available as film base by the roll. Thematerials formed by ALD, suitable for barriers, include oxides andnitrides of Groups IVB, VB, VIB, IIIA, and IVA of the Periodic Table andcombinations thereof. Of particular interest in this group are SiO₂,Al₂O₃, and Si₃N₄. One advantage of the oxides in this group is opticaltransparency which is attractive for electronic displays andphotovoltaic cells where visible light must either exit or enter thedevice. The nitrides of Si and Al are also transparent in the visiblespectrum.

The precursors used in the ALD process to form these barrier materialscan be selected from precursors known to those skilled in the art andtabulated in published references such as M. Leskela and M. Ritala, “ALDprecursor chemistry: Evolution and future challenges,” in Journal dePhysique IV, vol. 9, pp 837-852 (1999) and references therein.

The preferred range of substrate temperature for synthesizing thesebarrier coatings by ALD is 50° C.-250° C. Too high temperature (>250°C.) is incompatible with processing of temperature-sensitive plasticsubstrates, either because of chemical degradation of the plasticsubstrate or disruption of the ALD coating because of large dimensionalchanges of the substrate.

The preferred thickness range for barrier films is 2 nm-100 nm. A morepreferred range is 2-50 nm. Thinner layers will be more tolerant toflexing without causing the film to crack. This is extremely importantfor polymer substrates where flexibility is a desired property. Filmcracking will compromise barrier properties. Thin barrier films alsoincrease transparency in the cases of electronic devices where input oroutput of light is important. There may be a minimum thicknesscorresponding to continuous film coverage, for which all of theimperfections of the substrate are covered by the barrier film. For anearly defect-free substrate, the threshold thickness for good barrierproperties was estimated to be at least 2 nm, but may be as thick as 10nm.

Some oxide and nitride barrier layers coated by ALD may require a“starting” or “adhesion layer” to promote adhesion to the plasticsubstrate or the article requiring protection. The preferred thicknessof the adhesion layer is in the range of 1 nm-100 nm. The choice of thematerials for the adhesion layer will be from the same group of barriermaterials. Aluminum oxide and silicon oxide are preferred for theadhesion layer, which may also be deposited by ALD, although othermethods such as chemical and physical vapor deposition or otherdeposition methods known in the art may also be suitable.

The basic building block of the barrier structure is either: (A) asingle barrier layer with or without an adhesion layer, coated by ALD ona plastic or glass substrate, or (B) a barrier layer with or without anadhesion layer, coated by ALD on each side of a plastic substrate. Thisbasic structure can then be combined in any number of combinations bylaminating this building block to itself to form multiple, independentbarrier layers. It is known in the art of barrier coatings that multiplelayers, physically separate, can improve the overall barrier propertiesby much more than a simple multiplicative factor, corresponding thenumber of layers. This is demonstrated, for example, in J. Phys. Chem. B1997, vol. 101, pp 2259-2266, “Activated rate theory treatment of oxygenand water transport through silicon oxide/poly(ethylene terephthalate)composite barrier structures,” by Y. G. Tropsha and N. G. Harvey. Thisfollows because the path for diffusing gas molecules is tortuous throughmultiple barrier layers that are separated. The effective diffusion pathis much larger than the sum of the thickness of the individual layers.

Another barrier configuration involves directly coating the electronicor electro-optical device, requiring protection. In this regard, ALD isparticularly attractive because it forms a highly conformal coating.Therefore devices with complex topographies can be fully coated andprotected.

EXAMPLES Example 1

FIG. 1 shows a schematic representation of a light-emitting polymerdevice. For simplicity, the light emitting polymer device is shown asthe light-emitting polymer (LEP) sandwiched between two electrodes. Inpractice, a hole-conducting and/or electron-conducting layer can beinserted between the appropriate electrode and the LEP layer to increasedevice efficiency. The anode is a layer of indium-tin oxide and thecathode is a Ca/Al layer composite. With a voltage applied between theelectrodes, holes injected at the anode and electrons injected at thecathode combine to form excitons which decay radioactively, emittinglight from the LEP. The LEP is typically a photosensitive polymer suchas poly-phenylene vinylene (PPV) or its derivatives. The cathode isfrequently Ba or Ca and is extremely reactive with atmospheric gases,especially water vapor. Because of the use of these sensitive materials,the device packaging needs to exclude atmospheric gases in order toachieve reasonable device lifetimes. In FIG. 1, the package is comprisedof a barrier-substrate which can be plastic or glass on which the LEPdevice is deposited and then a top coated barrier film. The substrate iscomprised of a polyester film, polyethylene naphthalate (PEN) which is0.004 inch thick. Each side of the PEN film is coated with a 50 nm thickfilm of Al₂O₃, which is deposited by atomic layer deposition, usingtrimethylaluminum as the precursor for aluminum and ozone (O₃) as theoxidant. The substrate temperature during deposition is 150° C. In theALD process, the PEN substrate is placed in a vacuum chamber equippedwith a mechanical pump. The chamber is evacuated. The trimethylaluminumprecursor is admitted to the chamber at a pressure of 500 millitorr forapproximately 2 seconds. The chamber is then purged with argon forapproximately 2 seconds. The oxidant, ozone, is then admitted to thechamber at approximately 500 millitorr for approximately 2 seconds.Finally, the oxidant is purged with argon for approximately 2 seconds.This deposition process is repeated approximately 50 times to obtain acoating approximately 100 nanometers in thickness. The Al₂O₃ layer isoptically transparent in the visible. The coated substrate may be flexedwithout loss of the coating. One of the Al₂O₃ barriers is coated withindium-tin oxide transparent conductor by rf magnetron sputtering from a10% (by weight) Sn-doped indium oxide target. The ITO film thickness is150 nm. The LEP is spin coated on the ITO electrode, after which acathode of 5 nm Ca with about 1 μm of Al are thermally evaporated fromCa and Al metal sources, respectively. This LEP device is then coatedwith a 50 nm-thick, top barrier layer film of Al₂O₃, deposited by atomiclayer deposition, again using trimethylaluminum as the precursor foraluminum and ozone (O₃) as the oxidant. The resulting structure is nowimpervious to atmospheric gases.

Example 2

Another version of a packaging scheme is shown in FIG. 2. The top-coatedbarrier is replaced by an identical substrate barrier structure(Al₂O₃/PEN/Al₂O₃) without an ITO electrode as described in the Example 1above. This capping barrier structure is sealed to the substrate barrierusing a layer of epoxy.

Example 3

FIG. 3 illustrates a protection strategy with ALD barrier coatings foran organic transistor. The transistor shown is a bottom gate structurewith the organic semiconductor as the final or top layer. Because mostorganic semiconductors are air sensitive and prolonged exposure degradestheir properties, protection strategies are necessary. In FIG. 3 thepackage is comprised of a barrier-substrate on which the transistor isdeposited and then sealed to an identical capping barrier structure. Thesubstrate is comprised of a polyester film, polyethylene naphthalate(PEN), 0.004 inch thick. Each side of the PEN film is coated with a 50nm thick film of Al₂O₃, which is deposited by atomic layer deposition,using trimethylaluminum as the precursor for aluminum and ozone (O₃) asthe oxidant. The substrate temperature during deposition is 150° C. Inthe ALD process, the PEN substrate is placed in a vacuum chamberequipped with a mechanical pump. The chamber is evacuated. Thetrimethylaluminum precursor is admitted to the chamber at a pressure of500 millitorr for approximately 2 seconds. The chamber is then purgedwith argon for approximately 2 seconds. The oxidant, ozone, is thenadmitted to the chamber at approximately 500 millitorr for approximately2 seconds. Finally, the oxidant is purged with argon for approximately 2seconds. This deposition process is repeated approximately 50 times toobtain a coating approximately 100 nanometers in thickness. A gateelectrode of 100 nm thick Pd metal is ion-beam sputtered through ashadow mask on to the barrier film of Al₂O₃. A gate dielectric of 250 nmSi₃N₄ is then deposited by plasma-enhanced chemical vapor deposition,also through a mask to allow contact to the metal gate. This is followedby patterning of 100 nm-thick Pd source and drain electrodes, ion beamsputtered on the gate dielectric. Finally the top organic semiconductor,e.g. pentacene, is thermally evaporated through a shadow mask thatallows contact to source-drain electrodes. The entire transistor iscapped with an Al₂O₃/PEN/Al₂O₃ barrier-structure, sealed to substratebarrier with an epoxy sealant.

Example 4

In FIG. 4, the capping barrier of Example 3 can be replaced by a singlelayer of 50 nm-thick Al₂O₃, deposited by atomic layer deposition, usingtrimethylaluminum as the precursor for aluminum and ozone (O₃) as theoxidant. Both packaging structures for the organic transistor device areimpervious to atmospheric gases. The plastic substrate with barriercoatings can also be replaced by an impermeable glass substrate. Thebarrier capping layer is comprised of an initial adhesion layer ofsilicon nitride deposited by plasma-enhanced chemical vapor depositionat room temperature, followed by a 50 nm-thick Al203 barrier, depositedby atomic layer deposition, as described in Example 3.

Example 5

A substrate film of polyethylene terephthalate (PEN), 0.002 inchesthick, was coated by atomic layer deposition at 120° C. with Al₂O₃ about25 nm thick on one side of the PEN substrate. Prior to evaluating itspermeability properties the coated PEN substrate was flexed at leastonce to a radius of at least 1.5 inches to remove the coatedAl₂O₃-coated PEN substrate from the rigid silicon carrier wafer, towhich it was attached with Kapton® tape during ALD deposition. Theoxygen transport rate with 50% relative humidity was measured with acommercial instrument (MOCON Ox-Tran 2/20) through the film with Al₂O₃deposited by ALD. After 80 hours of measurement time, within themeasurement sensitivity (0.005 cc-O₂/m²/day), no oxygen transport(<0.005 cc/m²/day) through the barrier film was detected, in spite ofthe severe prior flexing. For comparison, we measured oxygen transportof about 10 cc-O₂/m2/day through an uncoated PEN substrate. FIG. 5 showsthat the optical transmission for this Al₂O₃-coated PEN barrier anduncoated PEN is the same (>80% transmittance above 400 nm) verifying thetransparency of the thin Al₂O₃ barrier coating.

1. An article comprising: a) a substrate made of a material selectedfrom the group consisting of plastic and glass, and b) a gas permeationbarrier deposited on said substrate by atomic layer deposition.
 2. Anarticle comprising: a) a substrate made of a material selected from thegroup consisting of plastic and glass, b) an adhesion layer coated, andc) a gas permeation barrier deposited an said substrate by atomic layerdeposition.
 3. An article comprising: a) a substrate made of a materialselected from the group consisting of plastic and glass, b) an organicsemiconductor, and c) a gas permeation barrier deposited by atomic layerdeposition.
 4. An article comprising: a) a substrate made of a materialselected from the group consisting of plastic and glass, b) a liquidcrystal polymer, and c) a gas permeation barrier deposited by atomiclayer deposition
 5. The article of any one of claims 1, 2, 3 or 4 wherethe article is an enclosed container.
 6. The article of any one ofclaims 1, 2, 3 or 4 where the article is an electrical or electronicdevice.
 7. The article of any one of claims 1, 2, 3 or 4 where thearticle is a light-emitting polymer device.
 8. The article of any one ofclaims 1, 2, 3 or 4 where the article is an organic light emittingdiode.
 9. The article of any one of claims 1, 2, 3 or 4 where thearticle is a transistor.
 10. The article of any one of claims 1, 2, 3 or4 where the article is a circuit comprising a light emitting polymerdevice.
 11. The article of any one of claims 1, 2, 3 or 4 where thearticle is a circuit comprising a transistor.
 12. The article of any oneof claims 1, 2, 3 or 4 wherein the article is an organic photovoltaiccell.
 13. The article of any one of claims 1, 2, 3 or 4 wherein the agas permeation barrier deposited by atomic layer deposition is depositedon a top side and a bottom side of the polymer.
 14. A second articlecomprising a plurality of layers, each layer comprising one article, asdescribed in any one of claims 1, 2, 3 or 4, wherein articles are incontact to each other.
 15. The second article of claim 11 wherein thearticles of any one of claims 1, 2, 3 or 4 are in contact with eachother by lamination means.
 16. The article of any one of claims 1, 2, 3or 4 wherein the article is a liquid crystal display.